1. Field of the Invention
The present invention relates to a wireless communication system for improving the performance of frame synchronization by using a training sequence. More particularly, the present invention relates to a wireless communication system for improving the performance of frame synchronization in an actual receiving environment by making up for characteristics that cause performance degradation of a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence due to a frequency offset. The present invention also relates to a method for forming a new training sequence for improving the performance of the frame synchronization in a wireless communication system.
2. Background Art
A wireless communication system usually includes a Base Station (BS) and a Mobile Station (MS) in order to support wireless communication services. The BS and MS support the services by using frames. They are required to acquire a mutual synchronization so as to transmit/receive frames. In order to acquire the mutual synchronization, the BS transfers a synchronizing signal so that the MS may detect the start of a frame that the BS transmits. In the wireless communication system, for acquiring the mutual synchronization in this manner, so as to precisely detect a starting point in time of the frame of a received signal, a training sequence is transmitted as the training sequence is arranged in the fore part of the frame. The training sequence corresponds to a signal of which a promise is made between a transmitter and a receiver. The receiver performs frame synchronization that detects the starting point in time of the frame by using a correlation pattern between the received signal and an original signal. A signal transmitted in a wireless communication environment can be distorted in various forms (e.g., a reflected wave, a diffracted wave, noise, multi-path interference, etc.) in the course of a transmission process. In addition, the signal transmitted can cause a problem such as a frequency offset due to an oscillator mismatch between the transmitter and the receiver. Therefore, it is necessarily demanded to design an adequate training sequence and a frame synchronization algorithm that can overcome the above problems.
CAZAC sequence among known training sequences has good properties of Peak-to-Average Power Radio (PAPR) and auto-correlation (refer to [1] R. L. Frank and S. A. Zadoff, “Phase Shift Pulse Codes with Good Periodic Correlation Properties,” IRE Trans. on IT, Vol. IT-7, pp 381-382, October 1962, [2] D. C. Chu, “Polyphase Codes with Good Periodic Correlation Properties,” IEEE Trans. on IT, Vol. IT-18, pp 531-532, July 1972, and [3] K. Fazel and S. Keiser, “Multi Carrier and Spread Spectrum Systems,” John Willey and Sons, 2003). Hence, CAZAC sequence is applied to a training sequence for frame synchronization in various systems, such as Orthogonal Frequency Division Multiplexing (OFDM), Single Carrier-Frequency Domain Equalization (SC-FDE), Ultra Wide Band (UWB), etc., ([4] refer to 3GPP TR 28.814, “Physical Layer Aspects for Evolved Universal Terrestrial Radio Access (UTRA)”).
Still, CAZAC sequence has the largest weak point in that CAZAC sequence cannot show a stable performance with respect to a frequency offset which can be incurred in the first stage of reception of a signal on characteristics of the signal itself. Namely, the larger the frequency offset becomes, the more conspicuously the performance degradation occurs. In a case where the frequency synchronization is not performed, it takes much time to detect the starting point in time of the frame. Especially, a stable frame synchronization cannot be performed in an environment where a large frequency offset can be generated, i.e., in a wireless communication system which has a low complexity and a low unit price, and in which the accuracy of an oscillator at the receiving end is not high.